Power generators, including nuclear reactors, are used for power generation, research and propulsion. A power generation circuit generally includes a heat source such as a nuclear core or furnace and a coolant circuit. Respective coolant piping circuits transport the heated water or steam to either a steam generator and then a turbine, or directly to a turbine, and after going through a condenser (heat sink), carries circulating or feedwater back to the heating source. Operating temperatures and pressure may range up to or above the critical point of water. Depending on the operational conditions, the various materials used must withstand the various load, environmental and radiation conditions.
Material used as coolant piping and other circuit and heat source components include but are not limited to carbon steel, stainless steels, nickel-based and other alloy steels and zirconium based alloys. These materials have to withstand the high temperature and high pressure condition. Although the materials have been carefully selected, corrosion occurs caused by the corrosive nature of high temperature, high pressure water, water radiolysis, and radiation effects. Such corrosion processes limit the lifetime of the boiling systems, and included but are not limited to stress corrosion cracking, flow accelerated corrosion, crevice corrosion and erosion corrosion.
Stress corrosion cracking (SCC), including intergranular stress corrosion cracking (IGSCC), is a well-known phenomenon happening to structural components in boiler coolant circuits, which affects the base and welding materials. SCC occurs through crack initiation, and propagation, which are caused by a combination of chemical, tensile and ductile stresses (static and dynamic). Such stresses are common in boiling environments caused by thermal expansion and contraction, residual stresses from welding, cold working, etc. The susceptibility toward SCC is often increased by the operating coolant environment, welding, heat treatment, radiolysis and radiation.
High oxygen content in the coolant has been shown to accelerate SCC through higher rates of crack initiation and propagation. High oxygen content in the coolant can stem from oxygen intrusion and water radiolysis processes, which create highly oxidizing species such as oxygen radical, hydrogen peroxide and many other radical species in the gamma, neutron, beta, and alpha flux.
The electrochemical potential (ECP) is a measure for the thermodynamic probability for corrosion to occur. The ECP is commonly employed to determine the rate of corrosion processes such as SCC, fatigue, film thickening and general corrosion. The ECP is directly proportional to the presence of oxidizing species such as oxygen, hydrogen peroxide, and any oxygen-containing radicals produced in the radiolytic decomposition of the boiling fluid and its additives.
The protection of unheated metal surfaces of nuclear reactors, which may be formed of steel and include boiler internals and piping of boiler systems, has been proved to be achieved at ECPs below −230 m V based on the standard hydrogen electrode (SHE) scale. The ECP in common water-cooled boiler systems is well above this threshold. As described for example in U.S. Pat. No. 6,793,883 and U.S. Pub. No. 2002/0118787, noble metals may be injected at varying intervals and concentrations to lower the ECP in boiler systems. The injection of noble metals may scavenge oxygen or oxygen radicals from the coolant and/or shift the water radiolytic decomposition equilibrium toward the recombination to water by catalysis.
The injection of noble metals may result in surface localized lowering of the ECP at unheated metal surfaces of nuclear reactors. The noble metals, which are injected as solution into the coolant, form partial or complete colloids or particulates under temperature, high pressure and radiation conditions. Thermal forces and electrostatic attraction of colloidal particles is the driving force for the deposition of the noble metal particles and colloids onto the coolant circuit surfaces to be protected and onto the surface of the heat source, i.e., a heat transfer surface, such as the cladding materials of nuclear fuel. The ECP of the surfaces is effectively lowered to below −230 mV caused by the noble metals catalyzed recombination reaction to water, which reduces the oxidizing species in the vicinity of the surfaces.
Boiling Water Reactors (BWRs), a subclass of water-moderated nuclear reactors, have employed noble metals injection for reactor internals stress corrosion cracking mitigation at unheated metal surfaces. In noble metal injection and application processes, dissolved noble metal (for example rhodium, platinum) solutions are injected into the reactor water systems.
The quantity of noble metals in coolant is high during the injections and decreases between injections. Noble metals not deposited on coolant circuit or heat source surfaces are typically removed by the coolant cleanup system. Different noble metal injection approaches have been developed, from once every two to three cycles, over annual injections, i.e. two injections per 24-month cycle of reactor operation (one at the beginning of the cycle, i.e. after 90 days, and another injections about 12 month after the first injection), to mini injections. Mini injections, i.e., about 1/10th of the typical annual injection rate, are injected on a monthly basis following an initial injection of about half the annual injection rate.
Corrosion products present in the coolant ultimately accumulate on the heat transfer surface, for instance on fuel element surfaces formed of zirconium, forming what is commonly called crud. The crud has a layer of low density loose crud, harboring mostly water, which is in constant exchange with the circulating reactor water, but providing a metal oxide structure capable of attracting and retaining colloidal particulates. This layer of low density loose crud is called fluffy crud. Below the layer of fluffy crud, closer to the heat transfer surface, a layer of higher density crud exists, called tenacious crud. The tenacious crud forms on a metal oxide layer of the heat transfer surface, which forms on heat transfer surface due to heating of heat transfer surface (i.e., general corrosion). For example, on fuel element surfaces formed of zirconium, heating results in the formation of a zirconium oxide layer. The fraction of tenacious crud increases as crud deposition increases and the crud ages. The densification is accelerated by excessive heat and prolonged exposure.
Noble metals have been detected and measured in crud deposits, even several cycles after the initial noble metals injections (in the case of injecting noble metals once every two to three cycles). The prolonged presence of noble metals in the crud deposits indicates that the noble metals are likely present in metallic particulate form, and are susceptible to redistribution throughout the reactor coolant system when the noble metals are present in the fluffy layer of the crud deposit. Noble metal particulates have also been identified in the tenacious crud fraction, close to the zirconium oxide layer of the element, which is generally a fuel rod or pin.
The sponge-like nature of the crud layer creates conditions corresponding to capillary water movement. The very low capillary velocities of fluids in crud, creating almost confined conditions, favor the water radiolysis reactions that form the molecular species, i.e. hydrogen, oxygen, hydrogen peroxide and the HO radical. Studies have shown that the hydrogen in confined spaces is ineffective in facilitating the recombination reaction to water. Hence, in confined spaces the sum of the oxidizing species, i.e. oxygen, hydrogen peroxide and oxygen radical, effectively create an oxygen saturated environment.
Metallic platinum and other noble metals are known to be electrochemical catalysts for both anodic hydrogen oxidation reaction (HOR) and cathodic oxygen reduction reaction (ORR). ORR rates are typically several orders of magnitude lower than the HOR rate. Platinum, however, when compared to other transition metals, has the highest ORR activity. Platinum alloys (PtNi, PtCr, PtFe, PtCo) have been shown to exhibit enhanced ORR activities compared to platinum alone.
Nano-sized platinum metallic particles have been shown to have an even higher catalytic activity when compared to smooth metallic surfaces, resulting from the available surface-to-mass ratio and crystallographic orientations. Furthermore, alloying platinum with transition metals has been shown to “extend” or enhance the catalytic properties. Studies have demonstrated that colloidal alloys of platinum with transition metals (V, Cr, Co, Ti, Ni, Zn) exhibit significantly higher electrocatalytic activities toward ORR than platinum alone. Dealloyed PtCu3 nanoparticles, for instance, have a higher catalytic activity than dealloyed PtCo3. This increase in catalytic activity toward the ORR has been explained by the ability of colloidal alloys of platinum with transition metals to break the O—O bond of O2 and to reduce the adsorbed atomic oxygen.